Graphene, a two-dimensional carbon material, can be useful in a wide variety of applications due to desirable properties such as chemical stability, mechanical strength, flexibility, transmittance, intrinsic carrier mobility, tunable band gap, and thermal and/or electrical conductivity. For example, graphene can potentially be used to replace conductive oxide films, e.g., indium tin oxide (ITO), in various devices such as displays, touch panel devices, or solar cells. Other useful applications for graphene films include field effect transistors, energy or thermal management devices, smart windows, and chemical or biological sensors. Certain applications call for transparent substrates coated with conductive films such as ITO or graphene, e.g., organic light-emitting diodes (OLEDs) or transparent conducting electrodes. Because graphene can remain conductive and stable even at the molecular level, graphene may play an important role in the next generation of electronics, e.g., enabling the development of smaller and/or lower power devices.
Several different methods have been employed for preparing graphene, including mechanical exfoliation from highly ordered pyrolytic graphite (HOPG), chemical reduction of graphite oxide, and high temperature annealing of single crystal silicon carbide. However, these methods may have various disadvantages in terms of scalability and/or manufacturing expense. Chemical vapor deposition (CVD) and plasma-enhanced CVD (PECVD) methods have shown promise as scalable and/or economical processes for growing graphene films on substrates. However, CVD typically utilizes a catalyst, such as Cu or Ni metals, and often requires additional steps such as transfer and/or catalyst removal steps. The CVD process may thus result in wrinkles, holes, and/or metal etching residues on the coated substrate, which may complicate subsequent applications.
The ability to successfully transfer synthesized graphene films onto a variety of substrates can be important for many end use applications. For example, if a user wants to flow electrical current through a graphene film, the conducting catalyst surface (or metal foil) should be removed and the graphene film transferred to an insulating substrate to avoid shorting out the device. Various methods have been developed for transferring graphene films onto different substrates, such as the use of polymethylmethacrylate (PMMA) or thermal release tape (TRT) to support the graphene film during transfer. For example, a transfer layer of PMMA or TRT can be applied to the graphene film to prevent folding, tearing, and/or wrinkling while the growth substrate or catalyst layer is etched away.
PMMA transfer methods can include, for instance, coating a layer of PMMA onto a graphene film on a growth substrate (e.g., metal substrate), etching or otherwise removing the growth substrate, and transferring the PMMA/graphene stack onto a target substrate. Solvents can be used to dissolve the PMMA and complete the graphene transfer. However, PMMA has a relatively low structural rigidity, which can make handling and transferring of large-scale graphene films difficult, if not impossible. TRT transfer methods include attaching a pre-cut TRT to a graphene film on a growth substrate, etching or otherwise removing the growth substrate, and transferring the TRT/graphene stack to a target substrate. Heat can be applied to reduce or eliminate the adhesion strength of the tape such that the TRT can be peeled off. While TRT transfer methods may be more easily scalable for production of larger graphene films, these methods may undesirably contaminate the graphene film surface with adhesive residues from the TRT. Such resides may be difficult to remove and/or the solvents used to remove the residues can adversely affect the electrical and/or optical performance of the graphene film. In addition, pressure applied to the TRT during the transfer process can also cause undesired deformations in the graphene film which may degrade the electrical and/or optical properties of the transferred film.
Accordingly, it would be advantageous to provide methods for transferring graphene films from a growth substrate to a target substrate without negatively impacting the physical, optical, and/or electrical properties of the transferred graphene film. Additionally, it would be advantageous to provide processes for the transfer of graphene films which are scalable and/or cost-effective. It would further be advantageous to provide transfer substrates comprising supported graphene films having improved properties.